Hydrocarbon gases and helium isotopes in the Paleozoic alkaline-ultramafic massifs of the Kola Peninsula

Hydrocarbon gases ( HCG) were studied in fluid inclusions from seven alkaline - ultramafic massifs of the Kola Peninsula. All the massifs (Sebljavr, Kovdor, Lesnaya Varaka, Ozernaya Varaka, Vuorijarvi, Turii Peninsula, and Salma) are central- type cratonic intrusions with ages of 360 - 410 Ma. Previous He isotopic investigations showed that the massifs have high He-3/He-4 ratios ( up to 3.3 x 10(-5)), which are usually higher than the upper mantle value. Similar to He, HCG were extracted by crushing. The HCG were analyzed for CH4 ( main component), C2H6, and C3H8. A comparison of HCG component contents with He isotope abundances and ratios showed that the HCG were probably not supplied by mantle-derived melts. Their formation during a postmagmatic stage at relatively low temperatures is our favored model.